Hi Safaa. Hemingway was an intelligent, gifted writer who was not very happy himself during his life.
Here he meant that if something valuable and highly prized is handed to you on a silver platter without any real merit (earning it), you have a tendency to appreciate and enjoy it less than if you worked very hard to achieve it. The same holds true with intelligence. And since personal happiness is based on our own individual perception of the world around us, it would stand to reason that possessing a supreme intelligence from birth would stand in the way of that happiness.Personally I think that there are many happy and intelligent people all around the world.